Known methods of producing a crystalline substrate having a concave-convex structure include (A) and (B).
(A) Lithography Technique
Lithography technique is a process wherein a resist is patterned by exposure to an energy ray irradiated through a mask formed with a pattern, and subsequent development.
Patterning of a resist film 18 by lithography technique comprises the step of applying a resist film 18 on the surface of a crystalline substrate 13 by spin coating wherein a coating solution is thinly spread by using centrifugal force (FIG. 8-a shows the state having the resist film 18 applied), and the step of exposure and development using an exposure apparatus (FIG. 8-b shows the crystalline substrate 13 having the patterned resist film 18 after the development). A crystalline substrate having a concave-convex structure 15 is formed by conducting etching of the crystalline substrate 13 using the resist film 18 which has been patterned by the lithography technique as a mask. The resist film 18 remaining on the surface of a crystalline substrate having a concave-convex structure 15 is subsequently removed (FIG. 8-c shows the crystalline substrate having a concave-convex structure 15 having the remaining resist film 18 removed). The crystalline substrate having a concave-convex structure thereon is thereby produced.
Precision in the patterning of the surface of a crystalline substrate used in a semiconductor device is generally determined by the minimum line width that can be formed on the resist film. The decrease in the minimum line width of the resist is known to be associated with a substantial decrease in the depth of focus of the exposure apparatus. The depth of focus is the range within which the optical system is substantially in focus.
This in turn means that, in the lithography technique, a decrease in the minimum line width of the resist patterning is associated with an uneven thickness of the resist film in the area where evenness of the crystalline substrate had been insufficient and, in such area, the surface of the resist film is no longer within the range of the depth of focus of the optical system of the exposure apparatus. The uneven thickness and width of the concave-convex structure of the resist film also results in the uneven height and width of the concave-convex structure formed on the surface of the crystalline substrate in the etching of the crystalline substrate, and yield and performance of the product are damaged in the area where the desired dimension has not been realized.
In addition, evenness of the crystalline substrate surface usually decreases with an increase in the diameter of the crystalline substrate, and as widely known, this in turn means that the increase in the diameter of the crystalline substrate inevitably invites adverse effects on the product yield and product performance. Typical indices for such surface evenness of the crystalline substrate are (i) TTV (total thickness variation) which is the proportion of the minimum thickness of the crystalline substrate in relation to the maximum thickness of the entire crystalline substrate, (ii) LTV (local thickness variation) which is the height of the local protrusion and recess in the crystalline substrate, and (iii) warp of the crystalline substrate (Single crystal wafers applied for surface acoustic wave (SAW) device applications—Specification and use guide, December 2005, Quartz Crystal Industry Association of Japan, pp. 5-6). For example, in the sapphire crystalline substrate which is commonly used for the growth of the light-emitting layer of a light-emitting diode, the thickness is 50 to 1000 μm, the TTV is 1 to 25 μm, the LTV is 1 to 15 μm, and the warp is −25 to 25 μm.
(B) Imprinting Technique
The imprinting technique is a process wherein patterning of the resist is accomplished by the very simple process of pressing of the mold. The imprinting technique includes thermal imprinting and UV (ultraviolet) imprinting.
The patterning of the resist film 18 by the thermal imprinting technique comprises the step of forming a resist film 18 having thermoplastic or thermosetting property by a spin coating method wherein the coating solution is thinly spread on the surface of the crystalline substrate 13 by using centrifugal force (FIG. 9-a shows the state having the resist film 18 applied), and the step of heating the mold and the resist film 18 to a temperature higher than the glass-transition temperature of the resist film 18 and pressing the mold against the resist film 18, and then cooling the mold and the resist film 18 to a temperature equal to or below the glass-transition temperature of the resist film 18, and releasing the mold from the resist film 18 (FIG. 9-b shows the crystalline substrate 13 provided with the patterned resist film 18 after the release of the mold).
The patterning of the resist film 18 by the UV (ultraviolet) imprinting technique comprises forming a resist film 18 having UV (ultraviolet) setting property by spin coating method wherein the coating solution is thinly spread on the surface of the crystalline substrate 13 by using centrifugal force (FIG. 9-a shows the state having the resist film 18 applied), and pressing the mold against the resist film 18 and irradiating the UV (ultraviolet) beam to cure the resist film 18 and releasing the mold from the resist film 18 (FIG. 9-b shows the crystalline substrate 13 provided with the patterned resist film 18 after the release of the mold).
The crystalline substrate having a concave-convex structure 15 is then produced by etching the crystalline substrate 13 using the resist film 18 patterned by the imprinting technique for the mask and, subsequently, the resist film 18 remaining on the surface of the crystalline substrate having a concave-convex structure 15 is removed (FIG. 9-c shows the crystalline substrate having a concave-convex structure 15 having the remaining resist film 18 removed). The crystalline substrate having a concave-convex structure is thereby produced (Japanese Unexamined Patent Publication (Kokai) No. 2009-54882). However, the imprinting technique has the drawback that the mold is incapable of following the crystalline substrate in the area where the evenness of the crystalline substrate is insufficient, and this invites uneven thickness of the resist film. The uneven thickness of the resist film means that, when the crystalline substrate is etched, the height of the concave-convex structure formed on the surface of the crystalline substrate becomes uneven, and that the yield and the performance of the product is reduced in the area where the desired dimension is not realized.
As described above, the evenness of the crystalline substrate has great influence on the yield and performance of the product in the production method of the crystalline substrate having a concave-convex structure in both of the methods (A) and (B).
In view of such situation, a technique of selectively polishing the protrusion of the crystalline substrate has been proposed.
More specifically, the technique is a process comprising the steps of detecting protrusions on the surface of the crystalline substrate and flattening the protrusions by local mechanical polishing, washing and drying the crystalline substrate for dust removal, and patterning the resist (Japanese Unexamined Patent Publication (Kokai) No. 2011-96935). However, the technique of selectively polishing the local protrusions on the crystalline substrate requires complicated and less-productive step of selective polishing of the protrusions on the crystalline substrate, and this technique can be used only in limited range of applications.
It could therefore be helpful to provide a method of producing a crystalline substrate having a concave-convex structure which can be conducted by a simple process, and which can be used even if the crystalline substrate has a large diameter.